As is well known, many pressure transducers generally employ piezoresistive elements which are disposed on a silicon sensor and which when subjected to a force or pressure exhibit a change in resistance.
Such devices have been used with oil which oil or fluid is employed as a force transmitting medium. In such devices the external pressure applied to the diaphragm or transducer is normally quite large. These devices frequently employ a metal diaphragm as a force collector, and a media isolator. The metal diaphragm communicates with a silicon pressure sensor through an oil filled reservoir which is contained in an internal hollow in the transducer housing.
For an example of a prior art device reference is made to U.S. Pat. No. 4,406,993 entitled "OIL-FILLED PRESSURE TRANSDUCERS" issued on Sep. 27, 1983 to Anthony D. Kurtz, the inventor herein, and assigned to Kulite Semiconductor Products, Inc., the assignee herein. Generally when silicon pressure sensor diaphragms are used to measure pressure in a relatively hostile environment, it is frequently necessary to isolate the silicon sensor from the pressure media by means of the metal isolation diaphragm as indicated. The media pressure is coupled to the silicon sensor by means of the oil layer which is located between the isolation diaphragm and the silicon sensor. This technique works very well for moderate pressures and temperatures. However, as the temperature increases, the oil will expand in volume. Unless the metal isolation diaphragm is not sufficiently compliant to deflect enough to accommodate the increase in oil volume, a thermally induced pressure will build up in the oil which will be transmitted to the silicon sensor. This effect gives rise to an error in the pressure measurement. For this reason, the metal diaphragm is made as compliant as possible and the oil volume is reduced as much as possible. The simplest way to increase the compliance of the metal diaphragm is to increase its radius and decrease its thickness since the compliance for any pressure is proportional to ##EQU1## where R is the radius and t is the thickness of the metal diaphragm. In addition, the expansion of the oil volume with temperature is proportional to the total oil volume. The easiest way to decrease the oil volume is to make the space between the metal diaphragm and the silicon sensor as small as possible. In most cases, by following the above rules the thermally induced error can be reduced to very low values and a very accurate transducer can be built.
However, when pressure measurements must be made very, very close to zero pressure, a new problem arises, which in fact is caused by obeying the aforestated rules. When the spacing between the metal isolation diaphragm and silicon sensor becomes too small (on the order of 0.010 inches) and the spacing between the isolation diaphragm and the mounting surface becomes on the order of 0.002 inches to 0.004 inches, surface tension at the metal diaphragm mounting surface through the oil medium and between the metal diaphragm and the silicon sensor through the oil medium can become significant. When the oil layer becomes sufficiently thin, then the surface tension between the metal diaphragm and the mounting surface and the metal diaphragm and the silicon becomes significant. As the pressure outside the metal increases, the diaphragm is steadily reduced, the surface tension causes the metal diaphragm to adhere to the mounting surface and to the silicon diaphragm. This places the oil in tension and causes the silicon diaphragm to deflect towards the metal diaphragm giving, in effect, a negative pressure reading.
It is therefore an object of the present invention to prevent this negative pressure reading while still allowing accurate pressure measurements.